Apparatus for regulating the toner concentration in a electrophotographic device

ABSTRACT

An apparatus in which the concentration of particles in a mix is controlled. The apparatus includes an electrode contacting the mix and electrically biased to attract particles thereto. After the particles are deposited on the electrode, it is illuminated with light rays. The intensity of the light rays transmitted from the electrode with the particles adhering thereto is indicative of the concentration of particles in the mix. This light ray intensity is detected. Thereafter, the electrode is discharged. A display shows when the electrical charge remaining thereon reached a preselected level.

[451 Mar. 25, 1975 Whited APPARATUS FOR REGULATING THE TONER CONCENTRATION IN A Primary Examiner-Fred L. Braun ELECTROPHOTOGRAPHIC DEVICE 75 Inventor: Charles A. Whited, Rochester NY X reen Xerox Corporation, Stamford, Conn.

[73] Assignee:

ABSTRACT [22] Filed: 1973 An apparatus in which the concentration of particles in a mix is controlled. The apparatus includes an electrode contacting the mix and electrically biased to attract particles thereto. After the Appl. No.: 416,154

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222/57 222/57 X 222/57 X 8 Claims, 3 Drawing Figures mow H M m tea tl Who hMm urta KTK 2 22 777 999 lll 78 332 753 542 378 6 6 6 333 PATENTED HAR 2 5575 saw 2 p g BACKGROUND OF THE INVENTION This invention relates generally to an electrostatographic printing machine and a development system employed therein. More particularly, the invention concerns an apparatus for controlling the concentration of particles in a mix used within the development system.

In electrostatographic printing, an electrostatic latent charge pattern is recorded and reproduced in viewable form. The electrostatic latent charge pattern corresponds to the original document to be reproduced. The field of electrostatography includes electrophotographic and electrographic printing. Electrophotographic printing is a class of electrostatography which employs a photosensitive medium to form, with the aid of radiation, the electrostatic latent charge pattern. Ele'ctrography is that class of electrostatographic printing which employs an insulating medium to form, without the aid of radiation, the electrostatic latent charge pattern. In all of the foregoing processes, the electrostatic latent image is developed or rendered visible by depositing the particles from a mix thereon in image configuration.

In electrophotographic printing, a charged photoconductive member is exposed to a light image of an original document to be reproduced. This records the electrostatic latent image on the photoconductive surface. A development system, thereupon, moves a developer mix of carrier granules and toner particles into contact with the electrostatic latent image. Toner particles are attracted electrostatically to the latent image forming a toner powder image thereon. The toner powder image is, then, transferred to a sheet of support material. Generally, the toner particles include heat settable resins. When such toner particles are transferred to the support material, the powder image can be permanently affixed thereto by the application of heat. It is fairly apparent from the foregoing description that the concentration of toner particles within the mix is determinative of the resultant image characteristics.

Multi-color electrophotographic printing substantially repeats the foregoing processes of charging, exposing, developing and transferring for a plurality of cycles. However, each development cycle deposits differently colored toner particles on the support material, in superimposed registration with the previously deposited layer of toner particles. Thus, the support material will have transferred thereto a multi-layer toner powder image.

The concentration of toner particles within the mix of the development system is regulated to obtain substantially optimum characteristics therefrom. Image density and color balance are maintained substantially constant from copy to copy by regulating the concentration of toner particles within the developer mix.

Various systems have been devised to adjust the concentration of toner particles within the developer mix. For example, U.S. Pat. No. 3,399,652 issued to Gawron in I968 discloses one such system. Gawron describes a rotating reflective disc positioned directly in the developer mix. An electrical bias is applied to the disc to attract toner particles thereto. The amount oftoner particles attracted to the surface of the disc is dependent upon the concentration thereof in the mix. A light beam is reflected from the surface of the disc onto a photoelectric unit. The intensity of the light rays transmitted to the photoelectric unit is a measure of the toner particles adhering to the disc surface. The photoelectric unit is calibrated to initiate a signal for dispensing particles to the mix when the density of toner particles adhering to the disc surface is less than a preselected minimum.

Another patent of interest is U.S. Pat. No. 3,754.82] issued to Whited in 1973. This patent discloses a transparent electrode positioned on a photoconductive drum and arranged to be electrically biased periodically to attract toner particles from the mix thereto during the development process. Light rays are transmitted through the transparent electrode. The intensity of the light rays passing through the transparent electrode having the toner particles adhering thereto is detected by a photosensor. The electrical output signal from the photosensor corresponds to the density of measured toner particles. This electrical output signal is compared to a reference signal. indicative of the desired toner particle density, and an error signal therebetween is developed. The error signal regulates the dispensing of additional toner particles to the development system.

None of the foregoing prior art devices appear to disclose any apparatus for returning the particle attracting disc or electrode adapted to a nominal value for successive cycles. Thus, a charge build up would possibly occur which could produce erroneous readings of toner particle concentration.

Accordingly, it is a primary object of the present invention to develop a control system wherein extraneous charges and residual charges are removed from the system.

SUMMARY OF THE INVENTION Briefly stated, and in accordance with the present invention, there is provided an apparatus for controlling the concentration of particles in a mix.

This is accomplished in the present instance by electrode means. Means are provided for periodically electrically biasing the electrode means to a first preselected level as the electrode contacts the mix. The electrically biased electrode means attracts particles thereto from the mix. Illuminating means project light rays onto the electrode means having the particles adhering thereto. Sensing means detect the intensity of light rays transmitted from the electrode means. An electrical output signal is developed by the sensing means which corresponds to the density of particles deposited on the electrode means. After the intensity of light rays has been detected, the electrical bias on the electrode means is discontinued and discharge means discharge the remaining charge thereon. Indicating means depict when the electrode means has been discharged to a second preselected level, and the foregoing process may be repeated for the next successive cycle.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which:

FIG. 1 is a schematic perspective view. of an electrophotographic printing machine incorporating the features of the present invention therein;

FIG. 2 is a sectional elevational view of the photoconductive drum and the control apparatus employed in the FIG. 1 printing machine; and a FIG. 3 is an electrical diagram depicting a portion of the electrical circuitry associated with the FIG. 2 control apparatus.

While the present invention will be described in connection with a preferred embodiment, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of themvention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION With continued reference to the drawings wherein like reference numerals have been used throughout to designate like elements, FIG. 1 schematically depicts a multi-color electrophotograp hic printing machine having the features of the present invention incorporated therein. The electrophotographic printing machine depicted in FIG. 1 showsthe various components employed therein for producing multi-color copies from a colored original. Although the control system of the present apparatus is particularly well adapted for use in conjunction with the development system of the FIG. I electrophotographic printing'machine, it should become evident from the following description that it is equally well-suited for use in a wide variety of electrostatographic printing machines and is not necessarily limited to the particular embodiment shown herein.

The printing machine depicted in FIG. 1 employs an image bearingmember having a drum mounted rotatably within the machine frame (not shown). Photoconductiv'e surface 12 is secured to and entrained about the exterior circumferential surface of drum 10. Preferably, photoconductivesurface 12 has a relatively panchromatic response to white light. One type of suitable photoconductive material is disclosed in U.S. Pat. No. 3,655,377 issued to Sechak in 1972. Drum 10 rotates in the direction of arrow 14 to move photoconductive surface 12 sequentiallythrough a series of processing stations. A drive motor (not shown) rotates drum 10 at a predetermined speed relative to the other operating mechanism. Various machine operations are coordinated with one another to produce the proper sequence of events at the appropriate processing stations. The sequence of events within the printing machine is controlled by the machine logic circuitry. The logic timing may be keyed from individual logic elements, or, in lieu thereof, keyed to the angular rotation of drum 10. In the event that the timing of the various sequence of events is keyed to the angular rotation of drum 10, a timing disc (not shown) may be mounted in the region of one end portion of drum 10. Preferably, the timing disc includes a plurality of angularly spaced slits in the periphery thereof. A light source is adapted to transmit light rays through the foregoing slits and energize a photosensor. Thus, as the various slits permit the passage of light therethrough, the photosensor is actuated to energize the appropriate logicci'rcuitry for initiating the various processes at therespective printing machine stations. I

Initially, drum 10 moves photoconductive surface 12 to charging station A. At charging station A, a corona generating device, indicated generally at 16, extends in a generally logitudinal direction transversely across photoconductive surface 12. This enables corona generating device 16 to readily charge photoconductive surface 12 to-arelatively high substantially uniform potential. One type of suitable corona generating device is described in U.S. Pat. No. 2,778,946 issued to Mayo in 1957.

Next, drum 10 is rotated to exposure station B where charged photoconductive surface 12 is exposed to a color filtered light image of the original document. Exposure station B includes thereata moving lens system, generally designated by the reference numeral 18, and a color filter mechanism, shown generally at 20. A suitable moving lens system is disclosed in U.S. Pat. No. 3,062,108 issued to Mayo in 1962. As illustrated in FIG. 1, an original document 22, such as a sheet of paper, book, or the like is placed face down upon transparent viewing platen 24. Lamp assembly 26, lens system l8, and filter mechanism 20 are moved in a timed relationship with drum 10 to scan successive incremental areas of original document 22 disposed upon platen 24. In this way, the optical system creates a flowing light image of original document 22 which is projected onto chargedphotoconductive surface 12. The irradiated areas of charged photoconductive surface 12 are discharged to record an electrostatic latent image corresponding to original document 22 thereon. Filter mechanism 20 is adapted to interpose selected color filters into the optical light path. The appropriate color filter operates on the light rays passing through lens 18 creating a single color light image. The single color light image forms an electrostatic latent image on photoconductive surface 12 which corresponds to a preselected spectral region of the electromagnetic wave spectrum, hereinafter referred to as a single color electrostatic latent image.

Drum 10, with the single color electrostatic latent image recorded on photoconductive surface 12, is, thereafter, rotated to development station C. Three individual developer units, generally indicated by the reference numerals 28, 30 and 32, respectively, are disposed at development station C. A suitable development station employing a plurality of developer units is described in copending application Ser. No. 255,259, filed on May 22, I972. Preferably, the developer units are all of a type referred to generally as magnetic brush developer units. A typical magnetic brush developer unit utilizes a magnetizable developer mix having carrier granules and toner particles. The developer mix is continually brought through a directional flux field to form a brush thereof. The electrostatic latent image recorded on photoconductive surface 12 is brought into contact therewith. Each of the respective developer units contain discretely colored toner particles corresponding to the complement of the spectral region of the wave length of light transmitted through filter 20, e.g., a green filtered electrostatic latent image is rendered visible with green absorbing magenta toner particles. Similarly, blue and red latent images are developed with yellow and cyan toner particles respectively. Each of the foregoing developer units contain its own supply of toner particles which are furnished to the developer mix as the concentration of toner particles therein is depleted beneath a pre-selected level as determined by the control apparatus of the present invention. Toner particle storage housing 34 is disposed in developer unit 28 so as to dispense additional magenta toner particles thereto when the developer mix thereof requires the same. Similarly, toner particle storage housing 36 is located in developer unit 30 and houses a supply of yellow toner particles which may be furnished to the respective developer mix as required. Finally, developer unit 32 contains toner particle storage housing 38 which supplies additional cyan toner particles thereto as required. After development, drum 10, with the toner powder image adhering electrostatically to photoconductive surface 12, is rotated to transfer station D.

At transfer station D, the toner powder image is transferred to a sheet of final support material 40. Final support material 40, may be, amongst others, plain paper or a thermoplastic sheet. A transfer roll, shown generally at 42, is located at transfer station D. Transfer roll 42 recirculates support material 40 and is biased electrically to a suitable potential and polarity to attract toner particles from the latent image recorded on photoconductive surface 12 thereto, A suitably electrically biased transfer roll is described in U.S. Pat. No. 3,612,677 issued to Langdon et al. in 1971. Transfer roll 42 rotates, in the direction of arrow 44, in synchronism with drum (in this case at the same angular velocity therewith). Since support material 40 is secured releasably on the exterior circumferential surface oftransfer roll 42 for movement in a recirculating path therewith, successive superimposed toner powder images may be transferred thereto.

Support material 40 is advanced from a stack thereof disposed on a tray. A feed roll, in operative communication with a retard roll, advances and separates the uppermost sheet from the stack. The advancing sheet moves into a chute which directs it into the nip ofa pair of register rolls. Thereafter, gripper fingers, mounted on transfer roll 42, secure releasably thereon support material 40 for movement in a recirculating path therewith. After a plurality of toner powder images have been deposited on support material 40, support material 40 is separated from transfer roll 42. The gripper fingers release support material 40 permitting a stripper bar to separate it from transfer roll 42. After support material 40 is stripped from transfer roll 42, and endless belt conveyor advances support material 42 to a fixing station (not shown).

At the fixing station, a fusing apparatus supplies sufficient heat to permanently affix the multi-layered toner powder image to support material 40. A fusing apparatus of this type is described in co-pending application Ser. No. 300,531 filed on Oct. 25, 1972, now U.S. Pat. No. 3,826,892. After the plurality of toner powder images has been affixed permanently to support material 40, support material'40 is advanced by a plurality of endless belt conveyors to a catch tray for subsequent removal therefrom by the machine operator.

With continued reference to FIG. 1, additional toner particles are added to the developer mix when the concentration therein is reduced deleteriously. The control apparatus, indicated generally at 46, includes a transparent electrode assembly 48 mounted on photoconductive surface 12 of drum l0. Illuminating means, such as light source 50, in cooperation with fiber optics 52 transmits light rays through transparent electrode assembly 48. During development, toner particles are deposited on transparent electrode 48 and the start intensity of the light rays transmitted thcrethrough is indicative of the density thereof. Sensing means. such as photosensor 56, is adapted to receive the light rays transmitted through transparent electrode assembly 48 and develops an electrical output signal corresponding to the intensity of the light rays. Comparing means, e.g. suitable analog and reference circuitry 54, compare the electrical output signal from photosensor 48 to a reference signal and generate a logic control signal for actuating the appropriate toner particles storage housing, i.e. housing 34, 36 or 38, to dispense the selected toner particles into the corresponding developer unit. A suitable recess is located in transfer roll 42 to prevent transparent electrode 48 from contacting transfer roll 42. In this manner, the toner particles deposited on transparent electrode 48 are not disturbed by the transfer process and represent a true indication of the concentration of toner particles within the developer mix.

The last processing station in the direction of drum rotation, as indicated by arrow 14, is cleaning station E. Although a preponderance of toner particles are transferred to support material 40, invariably some residual toner particles remain on pohtoconductive surface 12. These residual toner particles are removed from photoconductive surface 12 at cleaning station Ev As drum 10 moves through cleaning station E, the residual toner particles are initially brought under the influence of a cleaning corona generating device (not shown) adapted to neutralize the electrostatic charge remaining on photoconductive surface 12 and the toner particles. The neutralized toner particles are then cleaned from photoconductive surface 12 by a rotatably mounted fibrous brush 57 in contact therewith. A suitable brush cleaning device is described in U.S. Pat. No. 3,590,412 issued to Gerbasi in 1971.

It is believed that the foregoing description is sufficient for purposes of the present application to illustrate the general operation of a multi-color electrophotographic printing machine embodying the teachings of the present invention therein.

Turning now to FIG. 2, there is shown the detailed structural configuration of control apparatus 46. Control apparatus 46 includes a transparent electrode assembly 48, a light source 50, a fiber optic light pipe 52, a photosensor '56, and suitable logic circuitry 54 for processing the electrical output signal therefrom. Moreover, each of the developing units 28, 30 and 32, respectively, have a corresponding toner particle storage housing associated therewith, i.e. housing 34, 36 and 38, respectively. For example, toner particle storage housing 34 of developer unit 28 houses magenta toner particles, housing 36 of developer unit 30 stores yellow toner particles, and housing 38 ofdeveloper unit 32 stores cyan toner particles. Eachof the foregoing toner particle storage housings has perforations therein which are adapted to meter out a specified quantity of the selected toner particles to the appropriate developer unit. A suitable vibrator oscillates the appropriate toner particle storage housing to shear the toner particles, thereby dispensing toner particles through the perforations in the housing to the corresponding developer unit. Control apparatus 46 regulates the dispensing of toner particles from the toner particle storage housing to the respective developer unit.

Transparent electrode assembly 48 is mounted on photoconductive surface 12 of drum l0. Electrode assembly 48 is located in a non-image portion of photoconductive surface 12. As electrode assembly 48 passes through the development zone, the conductive surface thereof is biased with an electrical potential simulating the electrostatic latent image recorded on photoconductive surface 12. Preferably, electrode 48 is biased to about 200 volts above developer bias, normal developer bias being about 500 volts. However, electrode 48 may be biased from about 100 volts to about 600 volts above developer bias. The density of the image developed on electrode 48 is sensed by photosensor 56. The output signal from photosensor 56 is processed by logic circuitry 54 to develop an error signal. Photosensor 56 is mounted exterior to photoconductive surface 12 and positioned in a light receiving relationship with the light rays transmitted through electrode 48. Light source 50 may be inside drum 10 or, as shown in FIG. 2, external to drum 10 with the light rays conducted therein-by means of fiber optics 52. Shaft 58, supporting drum 10, is a tubular member and permits fiber optics 52 to pass through the hollow central core thereof and out therefrom to photoconductive surface 12 directing light rays from light source 50 to transparent electrode assembly 48.

, Transparent electrode assembly 48 is electrically bi ased to a suitable voltage level as hereinbefore indicated. The foregoing is achieved, preferably, by a commutator assembly, indicated generally at 60, positioned in the region of the end bell of drum 10. A suitable slip ring assembly may be used in lieu of commutator assembly 60. Timing for the application of the biasing voltage to electrode assembly 48 may be controlled by the use of a split commutator ring, i.e. electrode 48 being biased over one portion of the commutator and not over the remaining portion. When transparent electrode assembly 48 is not electrically biased, the residual charge remaining thereon is removed by the electrical circuitry depicted in FIG. 3. Light rays from transparent electrode assembly 48 are transmitted to photosensor 56 which is disposed in a suitable oven 62 via fiber optics 64. Fiber optics 64 is located in a chamber adapted to maintain the surface thereof substantially free from particle contamination. The detailed structure of configuration of the foregoing apparatus is described in US. Pat. No. 3,754,82l issued to Whited in 1973, the disclosure of which is hereby incorporated into the present application.

Referring now to FIG. 3, there is shown a sectional view of a portion of transparent electrode assembly 48. Transparent electrode assembly 48 includes a glass window having a transparent, tin oxide coating thereon. This type of transparent, electrically conductive glass is made by Pittsburgh Plate Glass Company under the trademark NESA or is made by the Corning Glass Company under the trademark Electro Conductor. The electrically conductive glass is mounted on brass ring 66 and is biased to an appropriate voltage level equivalent to the electrostatic latent image recorded on photoconductive surface 12 of drum 10. Brass conductor 66 is also electrically connected to neon lamp 68(Neon lamp 68 is electrically connected to resistor 70 which, in turn, is electrically connected to ground-In this mode of operation, when transparent electrode assembly 48 is electrically biased neon lamp 68 will be energized indicating that electrode 48 is electrically charged. When electrode 48 is not being biased, the extraneous charge remaining thereon is dissipated cycle. In addition, when electrode 48 has been discharged to a preselected level, i.e. to a level preferably of about 0 volts, neon lamp 68 no longer glows and the machine operator is advised that the electrode 48'has been returned to its initial uncharged state.

The apparatus described in FIG. 3 is also capable of indicating whether corona generating device 16 is functioning in its operative mode. By this it is meant that when transparent electrode 48 passes through the charging station A, it would be charged to a sufficient level to energize lamp 68. This would indicate to the machine operator that corona generating device 16 was operating in a satisfactory condition. By way of example, light source 68 is preferably a neon lamp while bleeder resistor 70 is preferably a resistor of about 2500 ohms. In this fashion, the extraneous charge remaining on the transparent electrode 48 is discharged therefrom. Neon lamp 68 functions as an indicator to inform the operator when residual charge has been removed from transparent electrode assembly 48. Indicating means on neon lamp 68 is operatively associated with discharge means or resistor 70 which is adapted to bleed the extraneous charge from electrode assembly 48. It should be noted that, if desirable, lamp 68 and resistor 70 may be electronically switched into electrical connection with brass connector 66 after the light rays have been transmitted through transparent electrode assembly 48 and prior to the cleaning operation thereof.

Thus, the control apparatus of the present invention is adapted to return to its initial state after each successive cycle to be capable ofbeing recharged to substantially the same level for the next successive cycle. Moreover, the readiness of the controlapparatus to be re-employed at the same charge levels is indicated by a suitable light display to the operator. In this manner, the control apparatus insures that the image density and color balance of the multi-color copy are repeatedly of a high quality.

While the present invention has been described as a discharge and display device, one skilled in the art will appreciate that the invention is not necessarily so limited and that the invention may also function as a failsafe device. For example, frequently the power supply energizing electrode 48 contains the requisite circuitry to discharge extraneous charge formed thereon. In the event that the circuitry contained in the power supply performing this operation should fail, the apparatus of the present invention would so indicate, and, in addition thereto, provide a back up by discharging residual charges remaining on electrode 48.

It is, therefore, apparent that there has been provided, in accordance with the present invention, an apparatus for controlling the concentration of toner particles within the developer mix that fully satisfies the objects, aims and advantages set forth above. While this invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

What is claimed is:

1. An apparatus for controlling the concentration of particles in a mix thereof, including:

electrode means; means for periodically electrically biasing said electrode means to a first preselected level as said electrode means contacts the mix to attract particles thereto;

means for illuminating said electrode means having the particles adhering thereto with light rays;

means for sensing the intensity of light rays transmitted from said electrode means to produce an electrical output signal corresponding to the density of particles deposited thereon; means for discharging said electrode means to a second preselected levelafter said sensing means detects the light rays transmitted therefrom; and

means, associated with said discharging means, for indicating that said electrode means is discharged to the second preselected level.

2. An apparatus as recited in claim 1, wherein said discharging means includes a resistor electrically connecting said electrode means to ground.

3. An apparatus as recited in claim 2, wherein said indicating means includes a lamp electrically connected in series with said resistor, said lamp being energized when said electrode means is electrically biased.

4. An apparatus as recited in claim 3, wherein:

said illuminating means includes a light source; and

said sensing means includes a photosensor positioned in a light receiving relationship with the light rays transmitted from said electrode means having the particles deposited thereon.

5. An electrostatographic printing machine of the type having a latent image recorded on an image bearing member and a development system employing a developer mix of carrier granules and toner particles to develop the latent image, wherein the improvement includes:

electrode means mounted on the image bearing member;

means for periodically electrically biasing said electrode means to a first preselected level as said electrode means contacts the developer mix to attract toner particles thereto; means for illuminating said electrode means having the toner particles adhering thereto with light rays;

means for sensing the intensity of light rays transmitted from said electrode means to produce an electrical output signal corresponding to the density of toner particles deposited thereon; means for discharging said electrode means to a sec ond preselected level after said sensing means detects the light rays transmitted therefrom; and

means, associated with said discharge means, for indicating that said electrode means is discharged to the second preselected level.

6. A printing machine as recited in claim 5, wherein said discharging means includes a resistor electrically connecting said electrode means to ground.

7. A printing machine as recited in claim 6, wherein said indicating means includes a lamp electrically connected in series with said resistor, said lamp being energized when said electrode means is electrically biased,

8. A printing machine as recited in claim 7, wherein:

said illuminating means includes a light source: and

said sensing means includes a photosensor positioned in a light receiving relationship with the light rays transmitted from said electrode means having the toner particles deposited thereon. 

1. An apparatus for controlling the concentration of particles in a mix thereof, including: electrode means; means for periodically electrically biasing said electrode means to a first preselected level as said electrode means contacts the mix to attract particles thereto; means for illuminating said electrode means having the particles adhering thereto with light rays; means for sensing the intensity of light rays transmitted from said electrode means to produce an electrical output signal corresponding to the density of particles deposited thereon; means for discharging said electrode means to a second preselected level after said sensing means detects the light rays transmitted therefrom; and means, associated with said discharging means, for indicating that said electrode means is discharged to the second preselected level.
 2. An apparatus as recited in claim 1, wherein said discharging means includes a resistor electrically connecting said electrode means to ground.
 3. An apparatus as recited in claim 2, wherein said indicating means includes a lamp electrically connected in series with said resistor, said lamp being energized when said electrode means is electrically biased.
 4. An apparatus as recited in claim 3, wherein: said illuminating means includes a light source; and said sensing means includes a photosensor positioned in a light receiving relationship with the light rays transmitted from said electrode means having the particles deposited thereon.
 5. An electrostatographic printing machine of the type having a latent image recorded on an image bearing member and a development system employing a developer mix of carrier granules and toner particles to develop the latent image, wherein the improvement includes: electrode means mounted on the image bearing member; means for periodically electrically biasing said electrode means to a first preselected level as said electrode means contacts the developer mix to attract toner particles thereto; means for illuminating said electrode means having the toner particles adhering thereto with light rays; means for sensing the intensity of light rays transmitted from said electrode means to produce an electrical output signal corresponding to the density of toner particles deposited thereon; means for discharging said electrode means to a second preselected level after said sensing means detects the light rays transmitted therefrom; and means, associated with said discharge means, for indicating that said electrode means is discharged to the second preselected level.
 6. A printing machine as recited in claim 5, wherein said discharging means includes a resistor electrically connecting said electrode means to ground.
 7. A printing machine as recited in claim 6, wherein said indicating means includes a lamp electrically connected in series with said resistor, said lamp being energized when said electrOde means is electrically biased.
 8. A printing machine as recited in claim 7, wherein: said illuminating means includes a light source; and said sensing means includes a photosensor positioned in a light receiving relationship with the light rays transmitted from said electrode means having the toner particles deposited thereon. 